ENGR2000_failure

ENGR2000_failure - Introduction to Engineering Materials...

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Unformatted text preview: Introduction to Engineering Materials ENGR2000 Chapter 8: Failure Dr. Priya Thamburaj Goeser Canopy fracture related to corrosion of the Al alloy used as a skin material. 8.2 Fundamentals of Fracture Fracture is the separation of a body into two or more pieces in response to an applied stress. The fracture process 1. Crack formation 2. Crack propagation Ductile Fracture Brittle Fracture Extensive plastic deformation in the vicinity of the crack Process proceeds slowly such that the crack length is extended Resists further extension unless there is an increase in the applied stress Stable crack Little plastic deformation Cracks spread rapidly Once started, will continue spontaneously without an increase in the applied stress. Unstable crack 8.3 & 8.4 Ductile & Brittle Fracture Extremely soft materials like gold & lead Ductile Fracture - Cup-and-cone fracture Ductile Vs. Brittle fracture 8.5 Principles of Fracture Mechanics Material properties Stress level Crack-producing flaws Crack propagation mechanisms The presence of a crack amplifies local stress in the vicinity of the flaw Consider two glass rods of equal dimensions, one of which has been scratched with a file. Which one is easier to break? the applied stress is amplified by the crack Stress Concentration Measured fracture strengths << theoretical predictions Caused by flaws or cracks at the surface or within the material The applied stress is amplified at the crack These flaws are called stress raisers Internal crack Surface crack Consider a crack similar to an elliptical hole through a plate and oriented perpendicular to the applied stress crack. internal an of length the half or crack surface a of length the is and crack tip,...
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ENGR2000_failure - Introduction to Engineering Materials...

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